1. Field of Invention
The present invention relates to a voltage reference circuit. More particularly, the present invention relates to a voltage reference circuit of a CMOS transistor.
2. Description of Related Art
FIG. 1 is a curve diagram of the relevant parameters of the semiconductor process technique. Supply voltage is scaling down because of reducing oxcide thickness. The threshold voltage (VTH) of MOS transistor, however, is not scale down as much as the supply voltage (VDD). Therefore, in the case that the voltage headroom is limited, all the analog circuits face the problem of maintaining their inherent capabilities under low operating voltage VDD.
FIG. 2 is a circuit diagram of a conventional voltage reference circuit. The PMOS transistors MP21 and MP22 biased in the sub-threshold region are adopted to successfully obtain larger voltage headroom, such that the circuit can operate under lower operating voltage VDD. The conventional voltage reference circuit includes a current mirror having PMOS transistors MP24˜MP26, PMOS transistors MP21˜MP23, an operation amplifier 201, and resistors R21, R22. For convenience of illustration, node voltages V21 and V22 are indicated. Then, the operating principle and the disadvantages of the conventional voltage reference circuit are illustrated with reference to FIG. 2.
As seen from the node voltages V21 and V22 of FIG. 2, by using the feedback mechanism formed by the operation amplifier 201 and the PMOS transistors MP24, MP25, the node voltage V21 is equal to the node voltage V22. Therefore, with the simple analysis of the circuit, the current I21 flowing through the resistor R21 is derived as follows.I21=(VSG21−VSG22)/R21  (1)
Herein, the current I21 is replicated to the resistor R22 through the current mirror, and the output reference voltage VBG is obtained as follows.VBG=VSG23+R22/R21*(VSG21−VSG22)  (2)
Since the PMOS transistors MP21, MP22 are biased in the subthreshold region in an area ratio of 1: K, by using the fact that current characteristics of the PMOS transistors, so they can be analyzed as bipolar transistors analysis, the reference voltage VBG is further expressed as
                              V          BG                =                              V            SG23                    +                                                    R                22                                            R                21                                      ·            n            ·                          V              T                        ·                          ln              ⁡                              (                K                )                                                                        (        3        )            where n is a process parameter, and VT is a thermal voltage. As seen from the formula (3), the conventional voltage reference circuit generates the temperature-independent reference voltage VBG by using the combination of the negative temperature coefficient voltage VSG23 and the positive temperature coefficient voltage VT.
Along with the changes of the circuit architecture, in order to allow the PMOS transistors MP21, MP 22 operate in the subthreshold region, the resistor R21 that is used by the conventional voltage reference circuit has a larger resistance value, and the current mirrors M24˜M26 may operate in the subthreshold region arises. Furthermore, the conventional voltage reference circuit outputs the reference voltage VBG, wherein the negative temperature coefficient voltage VSG23 is related to the negative temperature coefficient. Since the temperature coefficient of the current flowing through the PMOS transistor MP23 is in proportion to the absolute temperature, and additionally two input voltages (i.e. node voltages V21, V22) of the operation amplifier 201 are very small so that operation amplifier 201 may not operate at the low-gain region the present invention employs the resistors, R31 and R32 in FIG. 3, to increase the input common mode voltage of the operation amplifier, which will let opamp 311 operate at the higher gain region.